40 THE AGE OF THE EARTH 



are recognized as the ancestors of the higher Malaco- 

 straca, and yet these latter already existed in small 

 numbers, it is true side by side with the Phyllocarida 

 in the Devonian. The evolution of the one into the 

 other must have been much earlier. Here, as in the 

 Arthropoda, we have evidence of progressive evolution 

 among the highest groups of the Class, as we see in 

 the comparatively late development of the Brachyura 

 as compared with the Macrura. We find no trace of 

 the origin of the Class, or of the larger groups into 

 which it is divided, or, indeed, of the older among the 

 small groupings into families and genera. 1 



Of the Arachnida, although some of the most won- 

 derful examples of persistent types are to be found in 

 this class, but little can be said. Merely to state the 

 bare fact that three kinds of scorpion are found in the 

 Silurian, two Pedipalpi, eight scorpions, and two spiders 

 in the Carboniferous, is sufficient to show that the period 

 computed by geologists must be immensely extended 

 to account for the development of this Class alone, 

 inasmuch as it existed in a highly specialized condition 

 almost at the beginning of the fossiliferous series ; while, 

 as regards so extraordinarily complex an animal as a 

 scorpion, nothing apparent in the way of progressive 

 development has happened since. Professor Lankester 

 has, however, pointed out to me that the Silurian scorpion 

 Palaeophonus possessed heavier limbs than those of 

 existing species, and this is a point in favour of an 

 aquatic life like that of its near relation, Limulus. If 

 so, it is probable that it possessed external gills, not 

 yet introverted to form the lung-book. The Merostomata 

 are of course a Palaeozoic group, and reach their highest 

 known development at their first appearance in the 

 Silurian ; since then they have done nothing but dis- 

 appear gradually, leaving the single genus Limulus, 

 unmodified since its first appearance in the Trias, to 

 represent them. It is impossible to find clearer evidence 



1 For an account of the evolution of the Crustacea see the Presidential 

 Addresses to the Geological Society in 1895 and 1896 by Dr. Henry 

 Woodward. 



